Video monitoring of vehicle operators, such as car, truck or train drivers, aeroplane pilots and so on has been of interest for many years. As well as simply recording video of the operator to monitor their behaviour and state of alertness, various systems have been proposed for automatically detecting deteriorations in alertness caused by, for example, health problems or drowsiness. The results may be stored or, preferably, an alarm can be generated to alert the operator or the controls of the vehicle may be operated to bring the vehicle to a safe condition.
More recently, proposals have been made for analysing video images of human subjects and in particular analysing variations in skin tone in the image to detect a photoplethysmographic (PPGi) signal in the image representing the heart rate of the subject. Such a PPGi signal also carries a breathing rate modulation, allowing an estimate of both the heart rate and breathing rate in some circumstances. The video image of the subject can also be frequency analysed to detect movements in the expected frequency range for breathing rate or heart rate (such as movements of the chest or shoulders), and an estimate of breathing rate obtained from such movements. Such techniques are described in WO-A-2013/027027.
A problem, however, is that most demonstrations of the estimate of a subject's vital signs from video images are performed in very controlled conditions. Usually lighting is carefully arranged to be stable and even and the subject is requested to remain still during the period of video image capture. While the prior art demonstrates that it is possible to obtain reasonable estimates of heart rate and breathing rate in such conditions, in practical situations, particularly in moving vehicles, conditions are far from ideal and it is extremely difficult to obtain reasonable estimates of a subject's vital signs. In particular, illumination levels often vary spatially, while the motion of the vehicle induces motion in the subject, such that the level of illumination reaching a given region of the patient's skin varies temporally; even if the illumination levels relative to the car are temporally constant.
PPGi analysis techniques normally rely on the definition of one or more regions of interest on skin areas of the subject. The colour and/or luminance values from the regions of interest is used to provide a signal from which the PPG signal is to be obtained. However if the region of interest moves to a different skin region of the subject, this will introduce noise into the signal. Therefore it has been proposed to perform image registration on the video image so that the subject's position in the image is consistent (while the background appears to move), allowing the region of interest to be positioned at the same place within each image frame. However because the subject is moving within the spatially varying illumination field within the vehicle, although the subject's position in the image frame is stable, the brightness of the subject will seem to vary. Thus the movement of the subject will cause a variation in illumination which contaminates the image signal from which the PPG signal is to be obtained.
The same problem of a subject's movement causing a variation in illumination occurs in any video monitoring situation where the subject is moving and the illumination field is not completely spatially uniform.
An object of the invention is to provide for a method of processing a video image of a human subject to allow estimation of a vital sign, such as heart rate or breathing rate of the subject, which allows reduction of artifacts caused by motion-induced illumination variations and thus provides a better estimate of the vital sign.